1. Field of the Invention
The present invention relates to superplastic forming of multiple sheets in a manifold-free system, and more particularly to forming or shaping metal sheets with internally generated gas pressures.
2. Description of Related Art
Superplastic forming technology (SPF) has been frequently used in the aerospace industry to manufacture near net shape and stress free articles (i.e., components) through low strain rate forming operations under an applied pressure at elevated temperatures. Applied pressures and elevated temperatures have produced elongations up to 8000% or more in metals, up to 800% or more in intermetallics, up to 1400% or more in metallic composites, up to 1025% or more in ceramics, and up to 625% or more in ceramic composites. Recent SPF techniques involve laser welding or diffusion bonding to seal and join two or more sheets together in strategic locations so that when the assembly is pressurized with an inert gas at elevated temperature, the sheets inflate to fill the inside of a sealed die. After cooling, the manufactured component takes on the shape of the die, and may contain integrally stiffened members that are created when the strategically placed welds or bonds act as pinning points in the forming operation. Such multiple sheet SPF technology may show great promise at manufacturing complex shape structural components for the aerospace and other industries and has some advantages over conventional wrought metal forming processes and the like.
However, the commercial application of welded and SPF components has been economically limited, particularly due to high capital costs of SPF presses, to low throughput through the presses (e.g., batch modes) and by restrictions caused by connecting pre-inflated components to high pressure gas manifolds. Long forming times, on the order of hours, have discouraged improved SPF efforts.
In recent years, internally generated gas pressures have been employed to inflate malleable metal sheets. Trenkler et al. in U.S. Pat. No. 4,434,930 describe painting and sealing a pattern of thermally decomposable stop-off material onto an interfacial surface of two or more metal sheets, then solid phase green bonding the sheets and raising the temperature to decompose the stop-off material, thus generating gas and inflating the sheets contiguous to the pattern. However, such a technique suffers from employing inadequate and otherwise uncontrollable amounts of stop-off materials and consequently generating inadequate gas pressures. Oftentimes, the once-sealed painted patterns of the techniques such as those of Trenkler and others fail to provide additional stop-off material that can be added to generate additional internal gas pressure. Furthermore, the methods employing the once-sealed painted patterns have difficulty regulating the strain rate of inflatable superplastic materials and the like.
Accordingly, a need exists for more economical SPF methods that avoid batch processes and promote conveyor belt type processes, can avoid having to attach the components or articles to a high pressure manifold and can be manifold free, and offer flexibility in controlling or regulating fluid pressures during the formation of such articles.
The present invention relates to a method for forming a sheet into a desired shape by generating internal fluid pressures from a fluid-forming composition heated to a forming temperature in a container attached to the sheet. Novel articles are also produced by the invention. Such articles include (1) a container attached to at least two pre-inflated sheets, (2) shaped inflated sheets attached to the container, and (3) trimmed and finished, shaped inflated sheets for a desired use, such as aerospace components, vehicle exhaust manifolds, and the like.
In the method of the invention, a fluid, preferably a gas, is released from the heated fluid-forming composition in the container to exert gas pressure within an enclosed space between two or more heated adjacent, relatively flat sheets, e.g., materials exhibiting superplasticity, thereby inflating at least one of the sheets. An advantage of the invention is that the dimensions of a pathway between the fluid-forming composition in the container and the inflatable enclosed space of the desired article can be controlled to regulate gas flowrate to the enclosed space and thereby effect suitable strain rates to the sheet materials. Usually the container and any excess material is trimmed away from the resulting inflated article to produce a finished, or further-modifiable, inflated product. The method of the invention is useful as a continuous, rather than batch-type, process.
Inflated articles derived from the method of the invention can be utilized as intermediate or finished products. For instance, an article having a container (with or without the fluid-forming composition) attached to a pre-inflated sheet, can be transported to further manufacturers for article inflation. An inflated article attached to the container can be trimmed and finished at the location of formation or at a remote location therefrom.